This invention relates to a carburetor for small combustion engines and more particularly to a low speed fuel circuit to facilitate quick starting and warm-up of engines.
A small internal combustion engine requires extra fuel to run during xe2x80x9ccold startxe2x80x9d conditions. Traditionally, an automatic heat controlled choke is used on a diaphragm carburetor common with small engines. This choke blocks or restricts the air intake passage to the extent that the vacuum created by the moving piston within the engine will be higher than normal in the fuel-and-air mixing passage and thus will receive an increased quantity of fuel from the carburetor supply nozzle and delivers it to the engine cylinders. After the engine has started and has some time to develop heat, in the area of the automatic choke, there will be an automatic release of the choke to allow normal air flow into the mixing passage. These automatic chokes are expensive to manufacture and too costly for small engines.
With some small hand-held engines, such as chainsaws, weed cutters and/or trimmers, an extra quantity of fuel is forced into the engine by a manual priming pump or apparatus. This may facilitate the initial starting but usually will not provide sufficient fuel to keep the engine running until it warms up to the point that is needed to operate under normal carburetor conditions.
This invention provides a carburetor for a small engine capable of providing extra fuel for a cold start and cold running of an engine at idle conditions. A low speed fuel circuit has an air bleed line which communicates between an emulsification chamber and the inlet of a fuel-and-air mixing passage of the carburetor and is opened and closed by a restricting valve. A throttle valve is disposed rotatably within the mixing passage between a venturi and an outlet of the passage. The emulsification chamber has an outlet or low speed nozzle which communicates with the mixing passage downstream of the throttle valve when closed. Preferably, a low speed fuel flow control valve controls the amount of fuel entering the emulsification chamber, and a combination of the throttle valve and the air bleed shut off valve controls the amount of air which mixes in the emulsification chamber with the fuel required for engine idling conditions. When the engine is starting and idling cold, the restricting valve is closed manually and the emulsification chamber emits a rich mixture of fuel-and-air into the mixing passage downstream of the throttle valve. When the engine is starting and idling warm, the restricting valve is opened thereby providing additional air flow to the emulsification chamber for mixing with the fuel therein to produce a leaner fuel-and air-mixture emitted from the low speed nozzle. Preferably the restricting valve has a rotary shaft which may be mounted in the same location as a shaft of a common choke valve of a conventional carburetor.
Objects, features and advantages of this invention include providing a low speed circuit capable of flowing a richer fuel-and-air mixture to a small engine when the engine is starting and idling at cold conditions. The low speed circuit provides quicker cold engine start-UPS and significantly improves idling of the engine when cold. Because the restricting valve may replace a common choke shaft, this invention saves in manufacturing costs by reducing variability""s between carburetor models. The invention provides an extremely compact construction and arrangement, a relatively simply design, extremely low cost when mass produced, and is rugged, durable, reliable, requires little maintenance and adjustment in use, and in service has a long useful life.